Phase Behavior and Structural Ordering of Water under Superhydrophilic Confinement: A Molecular Dynamics Study using the mW Model

Abstract

We investigate how superhydrophilic confinement influences the thermodynamic and structural properties of water using molecular dynamics simulations with the coarse-grained monatomic water (mW) model. This work builds on our recent study of water confined within slit pores of varying widths and wall-fluid interaction strengths, focusing on properties such as density, enthalpy, potential energy, entropy, specific heat capacity, and structural ordering [1]. Strong wall-fluid interactions significantly alter phase behavior. In narrow pores (H ≤ 20 Å), the solid phase exhibits higher density than the liquid—an inversion of bulk water behavior—driven by confinement-induced structural ordering. Pronounced hysteresis loops are observed in thermodynamic quantities near transition temperatures, with their magnitude modulated by both pore size and wall interaction strength. Specific heat capacity displays enhanced oscillations due to non-uniform enthalpy changes across temperature intervals, and phase transitions are marked by abrupt shifts in both heat capacity and entropy. Structural ordering is characterized through in-plane orientational and tetrahedral order parameters, revealing the formation of ordered crystalline phases during quenching. These include cubic diamond, hexagonal diamond, and 2D hexagonal structures, whose prevalence varies with confinement conditions. Crystallization is most prominent under stronger surface attraction and narrower confinement, highlighting the role of nanoscale geometry in modulating water’s structural organization. These findings provide molecular-level insights into the behavior of confined water, with relevance to nanofluidic systems, biological interfaces, and porous materials. The use of the mW model enables efficient sampling of phase transitions while retaining essential physical characteristics of water. Our results also motivate future investigations that may incorporate multiscale modelling approaches to further explore confinement effects and phase behavior in complex environments.